Touch sensitive membrane switches have been incorporated into many electronic devices to enable operators to provide instructions to the device by selecting a corresponding horizontal and vertical coordinate location on the membrane switch. For example, membrane switches are often installed over the viewing screen of a cathode ray tube. The user of a device including such a "touch screen" is able to operate the device by pointing to and depressing a particular location on the screen corresponding to a desired menu selection. The touch screen then generates a voltage signal corresponding to the horizontal ("x") and vertical ("y") coordinates of that location. For such an application, the layers used to fabricate the membrane switch are transparent.
Other conventional applications for membrane switches are numeric and function keypads on diverse electronic items, such as microwaves, television sets, calculators, medical instrumentation, and various other devices. Membrane switches may be designed for manual finger or stylus depression for operation. The range of applications for membrane switches is ever increasing, as is the need for producing low cost membrane switches.
One type of conventional membrane switch, often used for touch sensitive screens, is the analog membrane switch. The membrane switch comprises a sandwich of top and bottom membranes with at least the top membrane being made from a flexible material. More typically, both membranes are made from flexible dielectric sheets. One surface of each membrane is coated with a semiconductive resistive layer, such as indium tin oxide ("ITO"), or a conductive layer such as gold.
To construct such a conventional analog membrane switch, top and bottom membrane sheets are etched to form an uncoated, dielectric border surrounding a semiconductive rectangle. Next, electrodes are applied to each of the top and bottom membranes, typically by silk-screening with a conductive ink. On the bottom membrane, two opposing parallel electrode strips are applied across first and second parallel edges of the semiconductive rectangle. On the top membrane, two opposing parallel electrode strips are applied across third and fourth parallel edges of the semiconductive rectangle. The electrodes on the top membrane are thus disposed perpendicular to the electrodes on the bottom membrane.
A layer of a dielectric material, such as an acrylic, is then applied over the top of the electrodes on each membrane. This prevents each set of electrodes from contacting the semiconductive rectangle or leads on the opposing membrane. A random or fixed array of small raised dielectric projections is then applied to the conductive-coated rectangle of the bottom membrane. Finally, the top and bottom membranes are cut, typically by die stamping, to remove excess sheet from around the electrode strips.
The top and bottom membranes are then assembled by superimposing the top membrane over the bottom membrane, with the conductive surfaces facing each other. An adhesive is applied between the borders of the membranes. The top and bottom membranes are normally maintained separate because of the presence of the array of dielectric projections. However, when the top membrane is depressed, it contacts the bottom membrane between the projections. The x and y coordinate locations of this point of depression can be obtained by monitoring voltage drops across the electrodes. Typically, a uniform potential, such as 5 volts, is first applied across a first set of electrodes formed on one of the membranes while the voltage drop across the second set of electrodes on the other membrane is monitored. This voltage corresponds to the horizontal, or "x" coordinate of the depression pointer. This arrangement is then switched, with a potential applied across the second set of electrodes and the voltage drop across the first set of electrodes being monitored to determine the vertical, or "y" coordinate. Monitoring of first and second sets of electrodes oscillates in this manner so that both the x and y coordinate of a depression point can be rapidly measured when such a depression occurs.
Construction and operation of conventional membrane switches is well known in the art, and is described in U.S. Pat. No. 3,522,664 to Lambright et al. Other voltage monitoring methods may be used to obtain similar results.
In addition to the above-described analog membrane switches, other configurations of membrane switches are well-known, such as four-wire digital membrane switches, three-wire membrane switches, and five-wire digital membrane switches. The main difference between these various versions are the number, configuration, and placement of the electrodes, as well as the monitoring methods and specificity of the coordinate measurements obtained. For any of these types, each membrane layer within the switch is subjected to at least the sequence of steps described above: etching to remove portions of the semiconductive coating; application of electrode strips; application of dielectric shield layers; and cutting to shape.
Each of these process steps requires handling of the unassembled top and bottom membranes, resulting in the potential for scratching or otherwise producing a defect in the semiconductive coating on the membranes. Any defects in the conductive coating results in a local variation in the resistive properties and inaccuracy in the coordinates location obtained. This problem is particularly pronounced for analog membrane switches, since analog switches are dependent on the linearity of the resistive semiconductive coatings to achieve high resolution. Thus, one small scratch or other defect on one of the membranes results in rejection of the membrane switch assembly. This problem is particularly pronounced for analog membrane switches due to the high resolution otherwise obtainable by such a switch. Conventional fabrication techniques result in on the order of a 50% rejection rate of analog membrane switches due to the high number of handling steps each membrane layer is exposed to, thus effectively doubling the cost of each membrane switch.